Wattages and Ohms Law for Electric Tankless Water Heaters

We sell Bosch Electric Tankless Water Heaters.

Electric Heaters and heating elements become hot due to a voltage being pushed through a wire.  The resistance in the wire measured in Ohms creates the heat.  The more resistance the hotter the wire gets. The more voltage pushed through the wire, the hotter the wire gets. Simply the smaller the wire (or the longer the wire run) the more resistance it gives to the voltage passing through.

So as voltage increases the wattage will increase four-fold.  As the voltage decreases the wattage will decrease by a factor of 4 as well.  The below table shows the percentage of drop or gain with different applied voltages to a heating element.

Rated at 277 volts the expected wattage % is

277 applied 100% (rated Wattage)

240 applied  75%

230 applied  69%

208 applied  56%

120 applied  19%

Rated at 240 volts the expected wattage % is

277 applied 133%

240 applied 100% (rated wattage)

230 applied  92%

208 applied  75%

120 applied  25%  (1/4 wattage)

Rated at 120 volts the expected wattage % is

240 Applied  400%  (4 times wattage - Dangerous) 

208 Applied  300%  (3 times wattage - Dangerous)

120 Applied  100%  (rated Wattage)

110 Applied   84%

This is all based on Ohm's Law.

Simple Ohms Law equations are:

Amps = WATTS / VOLTS

Watts = VOLTS x AMPS

Volts =  AMPS x OHMS

Ohms = VOLTS / AMPS

A Conductor Wire's resistance is constant based on wire size and length.

A heater's watt rating is based on the wire used in the heating element and the voltage applied.

To get a 9.5 kilowatt rated heater run at 240 volts; a wire with a resistance of 6.6 Ohms is used. (If there are two elements in the heater, then each is rated at 4.75 kw and the resistance of the wire would be 12.12.)

Calculating the Volume of Water in a Water Line

Whenever you turn on a gas, propane or electric tankless water heater the hot water starts heating up as soon as the minimum flow rate is met for the hot water demand. It takes a few seconds for the water to get heated up to the requested temperature. But, before the hot water starts to flow out of your shower or sink faucet the cold water that is in the pipe has to be pushed through your faucet. This is often referred to as pushing a "cold water slug".

Customers frequently ask us how much water is contained in this cold water slug and how long it will take to push this slug through their faucet. This can be easily calculated by using the formula for the volume of a cylinder.

The formula for the volume of a cylinder is V = Pi x r x r x h

where Pi = 3.1416; r = the radius of the water line; and h = height (or in this case the length) of the line

If your water line is 1/2" then r = .25" and r x r = .0625" and Pi x r x r = 0.19635 square inches
If your water line is 3/4" then r = .375" and r x r = .1406" and Pi x r x r = 0.4417875 square inches

Now to get all units in inches calculate h (the length of the water line) by taking feet x 12". As an example let's assume we have a 40' water line of 1/2" pipe. 40' = 480" so we calculate the Volume as:

V = 0.19635 square inches x 480" = 94.248 cubic inches

Now there are 1728 cubic inches in a cubic foot and there are 7.48 gallons in a cubit foot. Therefore there are 0.0043 gallons in a cubic inch.

So, 94.248 cubic inches * 0.0043 gallons = 0.4 gallons of water.

If you are running this cold water slug through a 1 GPM sink faucet then it will take .4 x 60 seconds or 24 seconds to push the cold water slug through the 1 GPM faucet. Add a few more seconds for the tankless water heater to bring the water up to temperature and remember that there are about 15' of internal pipe in a gas or propane tankless water heater that also starts out with cold water. In this example it will take just more that 1/2 minute to get the hot water flowing out of the faucet.

Bosch Neutralizers for Condensing Boilers and Water Heaters

Bosch recently has started to carry the Condensate Neutralizers for all the Bosch condensing boilers and Bosch tankless water heaters.  Condensate Neutralizers are recommended on all condensing equipment and should be serviced annually. Currently we offer 6 different JJM condensate neutralizers that cover the entire product range from the smallest output of 57,000 BTU (16kW) to the largest of 5,443,000 BTU (1,550kW).

Below is a list of what condensate neutralizer to use for each Bosch condensing boiler and Bosch tankless water heater.

The JM-2C & JM-2C PLUS can be used for any boiler or tankless water heater up to 200,000 BTU.

The JM-6C must be used for the Bosch C1210ES/ESC as it is over 200,000 BTU

Please call us at 866-289-4652 for more information and pricing.

Bosch Compress HP 200-1 Being Discontinued

Bosch has recently advised all wholesalers that the Bosch Compress HP 200-1 Heat Pump # 7736500041 is being discontinued.  Bosch is always looking for ways to improve products and while an analysis is conducted to look into options to offer a new model in 2015.  Support for the Compress HP 200-1 will still be available through Bosch Technical Support.  This item is currently out of stock.

Changes to Bosch Therm Outdoor Vent Kit - BTOK

Bosch is in the process of changing the color of all the Bosch Greentherm, Therm and Greenstar units to White from the old color of Silver.  This will effect the Bosch Outdoor Vent Kit - BTOK.

The new Bosch Outdoor Vent Kit - BTOK will change to white also.  Please see the new part number listed below.

 BTOK - #7736502448, (Old part #7709003213)

Changes to Bosch Therm Gas Tankless Water Heaters

Bosch is always upgrading their products and coming out with new and better models and changes.  Below are a few new changes to the Bosch Therm gas tankless water heaters.

The current silver-gray cover will be changing to Bosch white. This change is consistent with the Bosch residential product visual branding, such as with Greenstar, Greensource, Greentherm, and Therm.  Once the inventory of the units with silver covers is sold then Bosch will start changing over to the new white covers.

Warranty updates
For recirculation applications:
The C950ES, C1050ES and C1210ES heat exchanger warranty has been improved to 10 years for the following applications:
- Controlled recirculation applications
- Combination DHW and space heating applications
This warranty statement takes affect with products produced since April 2014 (FD 420). C950ES, C1050ES, and C1210ES products produced prior to April 2014 (before FD 420) will follow the previous warranty statement.

New changes to the Bosch Greentherm C950ES & C1050ES

Bosch is always making changes to their products to help improve performance.  Below are a few changes that have recently been approved for the Bosch Greentherm C950ES & Bosch Greentherm C1050ES tankless water heaters.

There have been a lot of new accessories added and approved for the heaters:
Accessories (Bosch part #)
• Concentric termination kit 3"/5"PVC (196006)
• Concentric vent kit 80/125 PP (7738003210)
• Concentric Up and Out kit (7 738 003 210)
• Neutralizer Kit (7738001483)
• Isolation Valve Kit (7738003449)
• Optional wireless remote control to operate with the appliance
(7709003213)
• Cascading kit (7 709 003 962)
• Outdoor kit (7709003913)
• Concentric termination kit 3"/6" PVC (196016)
• Exhaust/Intake bird screen (L2594)
• Condensate drain tee (196061) [included with Concentric
termination kits]
• Pipe Cover (PTPCES)
• High temperature kit (7736500074)
• Recess box kit (7736500043)
• Pressure relief valve (7 738 001 152)

The Bosch Greentherm C950ES & C1050ES can now be vented using 2" PVC/CPVC and must use the twin pipe system for this application.  Please see the venting section in the installation manual for more details.

The Front cover for the Bosch C950ES & C1050ES is changing from Silver to White.  This will be in effect for all the Greentherm and Therm units.

The Thermal Efficiency for the Greentherm C950ES heater is 94% with an Energy Factor of .95.
The Thermal Efficiency for the Greentherm C1050ES heater is 96% with an Energy Factor of .95.

Warranty updates
For recirculation applications:
The C950ES & C1050ES heat exchanger warranty has been improved to 10 years for the following applications:
- Controlled recirculation applications
- Combination DHW and space heating applications
This warranty statement takes affect with products produced since April 2014 (FD 420). C950ES & C1050ES products produced prior to April 2014 (before FD 420) will follow the previous warranty statement.